The present invention is directed to a new catalyst product and to a process of reducing organic compounds using said product. More specifically, the present invention is directed to porous base metal catalyst product having at least one precious transition metal dopant distributed on the surface area of the catalyst such that the surface to bulk ratio of dopant has a distinctly low value, as fully described herein below. The present doped catalyst product has been found to exhibit high catalytic activity and extended catalytic life compared to previously achieved values.
Hydrogenation catalysts based on highly porous nickel materials are well known. Such materials are part of a family of metal alloy derived products sold by W. R. Grace & Co.-Conn. under the trademark "Raney.RTM.". These porous materials, when microscopically viewed, take on a sponge-like appearance having tortuous pore channels throughout the nickel metal particle. Thus, such materials are generically viewed as porous or spongy metal alloy products. The metal alloy is generally composed of a major amount of a base metal selected from nickel, cobalt or copper with minor amounts of aluminum and other metals such as iron, chromium or molybdenum, as deemed appropriate for a particular application. The porous base metal catalyst product is normally referred to in terms of the metal which is the major component of the spongy metal product. These high surface area products have been found to have sites for hydrogen activation and, thus, exhibit catalytic activity when used in hydrogen reduction reactions.
It is known that the activity of spongy base metal catalysts can be enhanced ("promoted") by the incorporation of small amounts of certain transition metals. For example, French Patent 913,997 proposed incorporating chromium in up to 3.5 percent based on the content of nickel present in a Raney nickel catalyst. Promotion of catalysts was initially accomplished using transition metal elements which are readily available commodity metals, such as iron, molybdenum or chromium. These metals could be used in large amounts without causing a detrimental economic limitation to their commercial usefulness.
In general, porous base metal catalysts, such as porous nickel catalysts are formed by first producing a base metal-aluminum (preferred) or base metal-silicon alloy using conventional metallurgical techniques. The formed alloy is ground into a fine powder and classified by passing it through a sieve to provide a material having a desired particle size which is normally less than 500 microns and, preferably less than 75 microns. Larger particles are recycled for further grinding.
The alloy powder is then treated with a base to leach out a substantial amount of the aluminum metal or silica present. The base may be selected from either an inorganic (preferred) or organic compound. For example, in conventional processes an aqueous solution having from about 5 to 50 weight percent concentration of an alkali metal hydroxide (e.g., sodium hydroxide) is employed as the leaching agent. The treatment of the alloy is usually carried out at elevated temperatures of from about 40.degree. C. to 110.degree. C. The alloy powder can be directly added to the alkali solution or it can be formed into an aqueous suspension which is then contacted with the alkali solution. The aluminum contained in the alloy dissolves to form an alkali metal aluminate (e.g., sodium aluminate) with vigorous evolution of hydrogen. When silicon is in the alloy, the base forms the corresponding alkali metal silicate. The powder and alkali are normally allowed to remain in contact with each other for several hours at elevated temperature (e.g., 40.degree.-110.degree. C.) until the aluminum (or silicon) content is reduced to the desired level. The crude porous catalyst is separated from the reaction liquor and then conventionally washed with water until the wash water has a slightly alkaline pH value of about 8. The pore volume, pore size and surface area of the leached alloy will depend upon the amount of aluminum (or silicon) in the initial alloy and the degree of leaching. The nature of the porosity of the resultant base metal catalyst is one of tortuous pores throughout the volume of the catalyst particle. The resultant product normally has a pore volume (BET) of from about 0.05 to about 0.3 cc/g; an average pore diameter ranging from about 50 to 500 Angstroms; and a surface area (BET) of at least 10 m.sup.2 /g, preferably ranging from about 20 to about 150 m.sup.2 /g.
The resultant porous base metal product has been used as a hydrogenation catalyst to cause reduction of organic compounds, such as, for example, nitroorganics, to their corresponding amine compound. In order to further enhance the catalytic properties of such porous products, the addition of promoter metals, such as Group VIII transition metals (e.g., iron or chromium), has been previously accomplished by (i) adding the promoter metal to the base metal and aluminum (or silicon) when metallurgically producing the initial alloy; (ii) adding a salt of the promoter metal to the alkali leaching solution; or (iii) contacting the leached or leached and washed porous base metal catalyst with a salt solution of the promoter metal.
The process of adding promoter metal to the base metal during alloy formation, as disclosed in U.S. Pat. No. 3,781,227, has certain limitations. Firstly, it can be envisioned that some of the promoter metal is "encapsulated" in the solid body or skeleton of the base metal and not on the surface area of the resultant catalyst. In this form, the promoter metal does not cause a direct enhancement of the hydrogenation catalyst sites which are located on the surface area of the highly porous material. Further, a portion of the promoter metal may be removed during any one or all of the steps required to form the porous alloy. Thus, large amounts of a promoter metal are normally added during alloy formation to compensate for any loss during processing and through encapsulation. Because of the possible loss of promoter metal during processing and the inefficiency of encapsulated promoter metal, the alloy-addition method is not considered appropriate when the metal is a costly transition metal, such as platinum, palladium, osmium, ruthenium or the like.
Alternately, promoter metals have been added to the alkali leaching solution (see Great Britain Patent 1,119,512 and U.S. Pat. No. 3,326,725) in attempts to enhance resultant porous nickel's catalytic activity. The leaching solution is normally an alkaline aqueous or aqueous-alcoholic solution. In general, the promoter metal is introduced as an acid salt, such as a halide salt. In most instances, the leach solution does not maintain the promoter metal in solution but, instead, causes it to plate out on the outer shell of the porous base metal particle. Thus, the resultant porous particle has the promoter metal located on only a small fraction of the particle's surface area.
Spongy nickel or other base metal catalysts which have been previously formed and washed by conventional processes have been subjected to dopant metals just prior to use, in attempts to promote its catalytic activity. The dopant metal is normally introduced as an aqueous or aqueous/alcoholic solution of an acid salt, such as PtCl.sub.4, PdCl.sub.2, H.sub.2 PtCl.sub.6 or the like. In JACS 71 1515 (1949) and JACS 72 1190 (1950) Levering et al. disclosed the addition of an organic tertiary amine to the acid salt dopant solution. These authors taught that one should use the doped spongy metal product immediately after the addition of dopant (without further washing), in order to achieve enhanced catalytic performance. Such products exhibited only slight increase in catalyst activity and substantially no improvement in their active catalyst life.
It is highly desired to provide a promoted porous base metal catalyst (e.g., Raney.RTM. nickel) which exhibits high catalytic activity after storage (maintains good initial activity) and extended catalyst life during use (exhibits slow or delayed deactivation). Further, it is desired to provide a promoted base metal catalyst which has a precious transition metal as its promoter metal and said precious transition metal is substantially uniformly distributed as a coating on the surface area of the porous metal catalyst. Thus, the precious transition metal is substantially uniformly distributed across the particle diameter of said catalyst. Still further, it is desired to provide a precious transition metal promoted porous base metal catalyst wherein said promoter metal is present in up to about 1.5 percent by weight and the promoter metal's surface to bulk ratio (as defined herein beiow) is less than 60.